1. Field of the Invention
The present invention relates to a light emitting device, and more particularly, to an organic electroluminescence device.
2. Description of the Related Art
Among flat panel displays, an organic electroluminescence device (ELD) is a self-emission type display that has a high contrast and a wide viewing angle. The organic ELD can be made to be lightweight and have a slim profile as compared to other displays because it does not require a backlight. An organic electroluminescence device also uses less power than other types of flat panel displays. Further, the organic ELD can be driven with a low DC voltage and still have a rapid response time. Since all of the components of the organic ELD are formed of solid materials, it can withstand an impact. The organic ELD can operate throughout a wide temperature range and be manufactured at a low cost.
Unlike fabricating an LCD or a PDP, the organic ELD is manufactured by just using a deposition process and an encapsulation process. Thus, the manufacturing processes and apparatuses for making an organic ELD are very simple.
A passive matrix type organic ELD that does not have a switching element to drive the organic ELD has been widely used. In the case of the passive matrix type, gate lines (scan lines) cross data lines (signal lines) to define a matrix of sub-pixels. The gate lines are sequentially driven to drive each sub-pixel. In order to exhibit a required mean luminescence, a higher level of instantaneous luminance must be emitted sequentially in each sub-pixel across the display to create an overall average luminance.
In the case of an active matrix type, thin film transistors acting as switching elements are located in respective sub-pixels. First electrodes are connected to the thin film transistors, which are turned on/off by a sub-pixel unit. A second electrode facing the first electrodes is a common electrode.
The voltage applied to the sub-pixels charges a storage capacitor Cst so that the voltage can be applied until a next frame signal is applied, thereby continuously driving the organic ELD during one frame regardless of the number of gate lines. Accordingly, in the case of the active matrix type, even when low current is applied, uniform luminescence can be obtained. As a result, the organic ELD has the advantages of low power consumption, high definition and large-sized screen capability. Such an active matrix type organic electroluminescence device will now be described with reference to the accompanying drawing.
FIG. 1 shows a circuit diagram illustrating a basic sub-pixel structure of a related art active matrix type organic electroluminescence device. As shown in FIG. 1, gate lines (GL) 2 are formed in a first direction. Further, data lines (DL) 3 and power lines VDD 4 are formed in a second direction crossing the first direction to define a sub-pixel region. A switching TFT 5 is formed adjacent to a crossing of a gate line 2 and a data line 3. A storage capacitor CST 6 is connected to the switching TFT 5 and the power line 4. A driving TFT 7 connected to a current source element is connected to the storage capacitor CST 6 and the power line 4.
An organic electroluminescent diode 8 is connected to the driving TFT 7. When current is applied to the organic light emitting material in a forward direction, electrons and holes are recombined, moving through a P-N junction between an anode electrode as a hole donor and a cathode electrode as an electron donor. The energy of the organic electroluminescent diode 8 becomes lower than that created when the electrons are separated from the holes. This energy difference creates an emission of light. The organic electroluminescence device may be classified into a top emission type and a bottom emission type depending on which direction light is emitted from the organic electroluminescent diode.
FIG. 2 is a cross-sectional view of the related art bottom emission type organic electroluminescence device. As shown in FIG. 2, an organic electroluminescence device 10 includes a transparent first substrate 12, a TFT array 14 formed on the first substrate 12, a first electrode 16 formed over the TFT array 14, an organic luminescent layer 18 over the first electrode 16 and a second electrode 20 formed on the organic luminescent layer 18. The organic luminescent layer 18 reproduces red R, green G and blue B colors. For example, organic materials emitting R, G and B colors can be patterned in each sub-pixel P.
An absorbent material 22 is used to remove moisture and oxygen that may damage the organic electroluminescence device. A portion of the substrate 28 is etched and the absorbent material 22 is filled in the etched portion and fixed by a tape. The first substrate 12 is then adhered to the second substrate 28 by a sealant 26, thereby encapsulating the organic electroluminescence device.
FIG. 3 shows a sub-pixel of a TFT array included in the related art organic electroluminescence devices depicted in FIG. 2. In the case of the active matrix type organic electroluminescence device, each of the sub-pixels of the TFT array formed on the substrate is provided with a switching element TS, a driving element TD and a storage capacitor CST. Depending upon desired operation characteristics, the switching or driving transistors can be formed of a combination of more than one TFT. The substrate is formed of a transparent insulating material, such as glass or plastic.
As shown in FIG. 3, gate lines 32 and data lines 34 are provided in which the data lines 34 cross the gate lines 32. An insulating layer is formed between the gate lines 32 and the data lines 34. In addition, power lines 35 are formed in parallel with the data lines 34.
The switching TFT TS includes a gate electrode 36, an active layer 40, a source electrode 46, and a drain electrode 50. The driving TFT TD includes a gate electrode 38, an active layer 42, a source electrode 48, and a drain electrode 52. The gate electrode 36 of the switching TFT TS is connected to the gate line 32 and the source electrode 46 connected to the data line 34. The drain electrode 50 is connected to the gate electrode 38 of the driving TFT TD through a contact hole 54. The source electrode 48 of the driving TFT TD is connected to the power line 35 through a contact hole 56. Also, the drain electrode 52 is connected to the first electrode 16 formed on the pixel P.
In the related art bottom emission type organic electro luminescence device, the first substrate 12 on which the array element and the organic electroluminescent diode are formed is adhered to the separate second substrate 28 for encapsulation. In this case, the yield of the organic electroluminescence device is determined by multiplying the yield of the array element by the yield of the organic electroluminescent diode. Therefore, in the related art organic electroluminescence device, the entire process yield is greatly limited to a latter process, that is, the process of forming an organic electroluminescent diode. For example, even though the array element is formed excellently, if defects occur due to foreign matters or other factors in forming the organic electroluminescent layer using a thin film of about 1000 Å thick, the entire organic electroluminescence device is rendered defective.
A defective organic electroluminescent layer results in the loss of every expense and material cost spent in manufacturing the non-defective array element. The bottom emission type has high stability and high degree of freedom due to the encapsulation but has limitation in aperture ratio so that it is not applicable to high definition products. Further, when the voltage drop is small at the power line (4 in FIG. 1, 35 in FIG. 3) through which the voltage is supplied to each sub-pixel, the organic electroluminescence device can uniformly maintain the picture quality of the panel. However, in the case of the related art device shown in FIG. 3, there is a limit to the line width and thickness of the power line. Therefore, a voltage difference (voltage drop) occurs between the sub-pixels connected to the power line of the first stage and the sub-pixels connected to the power line of the last stage, such that a uniform picture quality cannot be obtained.